This invention relates to pneumatic tire molds and more particularly to the bead molding rings of pneumatic tire molds.
Pneumatic tires generally have two annular beads, each of which is comprised of at least one annular bead core. Bead cores are generally formed from a plurality of wound wires. When tires are built, the end portions of the carcass plies are firmly secured to the beads by wrapping them around the bead cores. Multiple bead cores are often used to increase tire stability in heavy duty tires such as aircraft, truck or off-the-road tires.
During vulcanization the beads of a tire are given a specifically designed profile which enables them to properly seat on the rim upon which the tire is intended to be mounted. In tubeless tires, the bead profile is especially critical because the sealing of air within the tire is dependent upon an airtight fit of the beads with the rim.
In operation, as each tread portion of a tire rotates through its footprint, the adjacent sidewalls bulge out. This bulging can cause relative movement between the bead and its associated rim portion. Bead movement can occur in the circumferential and axial directions relative to the rim. This movement may cause abrasion or chafing of the bead.
Excessive abrasion of the bead can cause the cords of the tire carcass to become exposed. If the cords are exposed, "air-wicking" may occur, whereby the pressurized air within the tire cavity passes internally through the ply cords to the sidewall and tread areas of the tire. The air will ultimately escape from the tire because the sidewall and tread rubber is not compounded to prevent air permeation.
To protect the ply cords from chafing and therefore to reduce the possibility of air-wicking, a finishing strip or chafer is often built into the bead of the tire. The finishing strip surrounds portions of the carcass plies which are wrapped around the bead core. Finishing strips are often of square woven nylon fabric which is specially treated to be nonair-wicking.
Excessive abrasive forces applied to the bead of a tire can also chafe a finishing strip if it is not well protected. To protect a finishing strip against deterioration due to chafing, it should be covered with a layer of tough, abrasive resistant rubber. The toe portion of the bead should especially be protected because most of the axial movement of the bead against the rim is concentrated inwardly of a hinge point near the toe of the bead. When building a tire, a layer of rubber can easily be applied at the bead over the finishing strip.
High compression forces are exerted upon the beads of a tire when it is vulcanized in a mold. The high molding compression can cause finishing strips and carcass plies to be pushed to the surface of the bead, thereby forcing some or all of the protective layer of rubber out of the mold. If a thin layer of the protective rubber remains covering the finishing strips and plies, some protection will be provided. However, a thicker layer of rubber is more desirable because it can absorb more deflection without deteriorating. A thin layer is more easily abraided and delaminated from the finishing strips. Delamination of the carcass plies can ultimately occur.
The beads of a tire must remain in good condition for the tire to remain operable. When the finishing strips and carcass plies deteriorate, the tire carcass is unusable and essentially irreparable. This can be a substantial loss, especially in heavy duty tires for use on aircraft, trucks, or off-the-road vehicles. These types of tire are generally retreaded several times.
Although abrasive forces are existent in virtually all types of tire, heavy duty tires are especially affected. Heavy duty tires often operate under severe conditions which can accelerate bead chafing. Such operating conditions include excessive braking head and heavy loads that can cause excessive deflection. A thicker layer of protective rubber is therefore required in these types of tires.
Some heavy duty tires may have four to twelve carcass plies which are wrapped around a bead core, thereby creating a large volume of material underneath the bead core that must be compressed during vulcanization. This can compound the aforementioned problem of high compressive forces in the bead area during vulcanization.
Prior attempts at maintaining a protective layer of rubber over the fabric at the beads of a tire have included the use of several rubber compounds. However, no compound has been found to completely eliminate the flowing of rubber under the high compression forces applied during vulcanization. Semi-cured compounds have been suggested, but they are difficult to handle in a factory setting, difficult to age control, and difficult to adhere to the tire. Larger beads and lower molding compressions have also been proposed. Neither of these proposals have provided satisfactory results, primarily because of the critical function of the beads and the resulting specificity required in bead design.